The term "light hydrocarbon" is used herein to mean a hydrocarbon of from 1 to about 3 carbon atoms. Included are methane, ethane, ethylene, acetylene, propane, propylene and the like as well as mixtures of two or more thereof. The term "liquid hydrocarbon" refers to hydrocarbons that are substantially in the liquid form at a temperature of about 25.degree. C. and a pressure of one atmosphere.
Natural gas typically contains about 40-95% methane depending on the particular source. Other constituents include about 10% of ethane with the balance being made up of CO.sub.2 and smaller amounts of propane, the butanes, the pentanes, nitrogen, etc. Primary sources for natural gas are the porous reservoirs generally associated with crude oil reserves. From these sources come most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter.
Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, the butanes, the pentanes, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 70% to more than about 95% by volume of methane. Processed natural gas is used principally as a source of heat in residential, commercial and industrial service.
Most procesed natural gas is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decrease, new sources that are more distant require additional transportation. Many of these distant sources are not, however, amenable to transport by pipeline. For example, sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water are not amenable to transport by pipeline. This problem has been addressed in several ways.
One approach has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited as the natural gas can be used only for one product, preempting other feasible uses.
Another approach has been to liquefy the natural gas using cryogenic techniques and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by such cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas to a temperature of about -162.degree. C., transporting the gas, and revaporizing it are complex and energy intensive.
Another approach is to convert lower hydrocarbons (e.g., methane, natural gas) to C.sub.2 + hydrocarbons using an arc discharge or an arc plasma process. With arc discharge, the feedstock is fed into the reactor to intersect with an electric arc struck between a graphite cathode and a metal (copper) anode. The reaction temperature is about 1500.degree. C. with residence times of a few milliseconds before the reaction temperature is drastically reduced by quenching with water. Arc plasma is similar to arc discharge except that an auxiliary gas (e.g., hydrogen) is used as a heat carrier. The first successful commercial installation for electric arc cracking of lower hydrocarbons to acetylene was built by I.G. Farbenindustrie in 1940 at Huels. Other commercial arc discharge processes that have been installed were those of the DuPont Company which used a high-speed rotating arc and a Romanian process which produces ethylene and acetylene. The Huels and Romanian processes are believed to be still operating. The DuPont process was shut down in 1969. The arc plasma process has also been developed to industrial scale. See, Kirk-Othmer, "Encyclopedia of Chemical Technology", Third Edition, Volume 1 at pp. 214-218. A disadvantage with arc discharge is that large amounts of carbon are formed. An advantage of the arc plasma process is that acetylene can be produced from heavy feedstocks without the excessive carbon formation of a straight arc discharge process.
Still another approach involves the use of pyrolysis to convert methane and/or natural gas to higher molecular weight hydrocarbons (e.g., substantially liquid hydrocarbons) that can be easily handled and transported. Low temperature pyrolysis (e.g., to 250.degree. C. and 500.degree. C.) of hydrocarbon feedstocks to higher molecular weight hydrocarbons is described in U.S. Pat. Nos. 4,433,192; 4,497,970; and 4,513,164. The processes described in these patents utilize heterogeneous systems and solid acid catalysts. In addition to the solid acid catalysts, the reaction mixtures described in the '970 and '164 patents include oxidizing agents. Among the oxidizing agents disclosed are air, O.sub.2 -O.sub.3 mixtures, S, Se, SO.sub.3, N.sub.2 O, NO, NO.sub.3, F, etc. The conversion of methane and/or natural gas to higher molecular weight hydrocarbons at higher temperatures (e.g., above about 1200.degree. C.) using pyrolysis has been suggested. These high-temperature processes are, however, energy intensive and have thus far not been developed to the point where high yields are obtained even with the use of catalysts. Some catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
A common technique for pyrolyzing methane and/or natural gas involves the use of tubular reactors. The methane or natural gas flows through a tube placed inside a radiant and/or convective chamber of a furnace. The heat supplied to the methane or natural gas is dependent upon the surface area of the tubes, and thus only relatively small diameter tubes are typically used. During such pyrolysis, carbon tends to build up on the inner walls of the tubes. Because of the small diameter of the tubes, any deposited carbon forms a relatively thick layer and thereby severely inhibits further heat transfer. Tubular reactors can be used for cracking hydrocarbons like ethane or propane due to the fact that hydrocarbons of this type do not produce significant levels of carbon. However, the amount of carbon produced during the pyrolysis of methane or natural gas is substantially greater and thus to date it has not been feasible to pyrolyze methane or natural gas in these reactors for more than a few minutes or a few hours at a time because of carbon build-up on the inner walls of the tubes.
Stanley, H. M., et al, in "The Production of Gaseous, Liquid, and Solid Hydrocarbons from Methane. Part I--The Thermal Decomposition of Methane", Transactions, Journal of the Society of Chemical Industry, Jan. 11, 1929, Vol. 48, pp. 1-8, disclose that in the pyrolysis of methane the use of relatively long heating periods tends to cause methane to decompose into its elements (i.e., carbon and hydrogen) almost exclusively, and that such decomposition produces an accumulation of carbon that is unfavorable to the production of good yields of higher hydrocarbons. They indicate that this tendency is greatly increased by the use of large heating surfaces and by the presence of active materials such as nickel and iron. The solution they suggest is to use short periods of heating, as low as 0.4 second (400 milliseconds), temperatures of 1000.degree.-1200.degree. C., and relatively inactive heating surfaces, such as silica. They indicate that under such circumstances methane decomposes to produce numerous products including acetylene, ethylene, ethane, higher olefins, benzene and higher aromatic hydrocarbons, and that the production of carbon and hydrogen may become almost negligible.
U.S. Pat. No. 3,093,697 discloses a process for making acetylene by heating a mixture of hydrogen and a hydrocarbon stock (e.g., methane) at a reaction temperature that is dependent upon the particular hydrocarbon employed for about 0.01 to 0.05 second (10 to 50 milliseconds). The reference indicates that a reaction temperature of 2700.degree. F. (1482.degree. C.) to 2800.degree. F. (1538.degree. C.) is preferred for methane and that lower temperatures are preferred for higher molecular weight hydrocarbons. The molar ratio of hydrogen to hydrocarbon stock is between about 1 to 8 moles of hydrogen for each carbon atom of the hydrocarbon molecule.
U.S. Pat. No. 3,156,733 discloses a process for the pyrolysis of methane to acetylene and hydrogen. The process involves heating a methane-containing stream in a pyrolytic reaction zone at a maximum temperature above 1500.degree. C. and sequentially withdrawing a gaseous product from said reaction zone and quenching said product rapidly to a temperature of about 600.degree. C. or less.
U.S. Pat. No. 4,176,045 discloses a process for the production of olefins by steam-cracking normally liquid hydrocarbons in a tubular reactor wherein the residence time in the tubes is from about 0.02 to about 0.2 second (about 20 to about 200 milliseconds) and the formation of coke deposits in the tubular reactor is minimized.
U.S. Pat. No. 4,479,869 discloses a process for preheating a hydrocarbon feed (e.g., ethane, propane or mixtures thereof) to a steam-cracking furnace in a tubular reactor wherein the hydrocarbon feed is heated within the temperature range of about 370.degree. C. to about 700.degree. C. by indirect heat exchange with super-heated steam. The reference indicates that such preheating reduces the propensity for coke deposition and permits the production of ethylene from a wide range of feeds.
U.S. Pat. No. 4,520,217 discloses a process for producing light aromatics from a feedstock comprising one or more of the natural gas liquid components which comprises: (1) pyrolyzing the feedstock in a first pyrolysis zone at a temperature of about 620.degree. C. to a temperature in excess of about 750.degree. C. for less than about one second (1000 milliseconds); (2) admixing the pyrolyzed feedstock with a pyrolyzed recycle stream comprising C.sub.2-4 hydrocarbons such that the sensible heat of the admixture is sufficient to initiate the reaction of forming light aromatics; (3) quenching the reacting admixture; (4) separating the reacted admixture into a C.sub.8 + tar and an offgas; (5) further separating the offgas into fractions comprising a light aromatic product and the recycle stream; and (6) pyrolyzing the recycle stream in a second pyrolysis zone to form the pyrolyzed recycle stream of step (2).
Chemical Economy and Engineering Review, July/August 1985, Vol. 17, No. 7.8 (No. 190), pp. 47-48, discloses that furnaces have been developed commercially for steam cracking a wide range of liquid hydrocarbon feedstocks using process reaction times in the range of 0.05 to 0.1 second (50-100 milliseconds). This publication indicates that the use of these furnaces permits substantial increases in the yield of olefins--ethylene, propylene, butadiene--while decreasing production of less-desirable co-products.